Key trends in satellite communications include the use of digital beam-forming architecture within the payload design and the provision of coverage over global regions in the form of narrow spot beams. Multiple spot beam coverage provides advantages of gain and frequency reuse. It is known accurately to define such narrow spot beams, with a multi-element antenna system, by digital beam-forming techniques involving assigning complex digital weights to each communication frequency channel for controlling spot beam parameters: see A. M. Bishop et al “The INMARSAT 4 Digital Processor and Next Generation Developments”, 23rd AIAA ICSSC, Rome, Italy, September 2005. Further improvements are nevertheless desirable, particularly in accuracy of beam-pointing, which is at present only nominally accurate, since beam-pointing errors may significantly affect gain.
As a technique, it is well-known to provide ground beacons for providing a satellite with reference signals from accurately known positions, enabling the satellite to measure pointing error relative to the beacons by means of an RF sensing system and to correct the pointing error by adjustment of its attitude and/or antenna orientation.